New option --dist for build.sh

* po/Makefile.am (EXTRA_DIST): Remove removed file.
* Makefile.am (nightly:): Remove target.
* build.sh (usage): New option --dist.

We also fix a minor dist bug here and prepare for a nightly option in
build.sh. Having the nightly target in the Makefile does not make much
sense because the Makefile does not initially exist.